1. Field of the Invention
The invention relates to a pulse-width-modulated current control circuit for driving inductive loads in motor vehicles and to its use.
2. Background of the Invention
Electronic motor vehicle control units, such as controllers for ABS and/or ESP motor vehicle brake control units, comprise multiply redundant microprocessor systems and additional power circuits for driving loads such as, for example, the electromagnetic valve solenoids which are necessary for regulating pressure. Modern electronic brake control units for brakes now only comprise for this purpose a limited number of highly integrated components in which most of the discrete components of the controller are combined in two integrated modules, or even just one integrated module. An integration stage which is customary nowadays comprises two integrated circuits, with the microcomputer systems being combined in a first component and the power circuits being combined in a second, mixed analog/digital circuit. In the second integrated circuit there is also an analog/digital converter making available the analog value for the microcontroller as digital values. For reasons of cost it is advantageous to use a single A/D converter for a plurality of measurements.
In high-quality electronic ABS and ESP brake control systems, the valve solenoids are, at least partially, no longer switched but rather analogized driving is carried out by means of a pulse-width-modulated current controller (PWM) which permits virtually analog driving of the hydraulic valves. For this purpose, multi-channel PWM driver stages are provided which can be constructed, for example, by means of MOS transistors which are switched in antiphase. In order to permit an economic and space-saving solution, such a PWM stage is usually implemented as an integrated circuit, especially since up to eight of such stages have to be present for a complex ESP system as well as numerous additional circuit components. A pure analog amplifier for driving a valve solenoid is not practical because of an excessively high power loss.
The basic procedure when using a single A/D converter for measuring the actual current within a PWM controller for driving the abovementioned valve solenoids is already known from WO 02/058967 A2 (P 10057) and WO 03/039904 A2 (P 10253). According to the circuit examples described therein, a specific number of current-measuring channels are assigned to the A/D converter in accordance with a complex priority logic corresponding to a time slice principle so that its conversion capacity can be used in the best way possible.
The requirements which are made of the above electronic control units are continually increasing since additional functions are also performed by the brake control unit and the brake systems are intended to exhibit improved control quality. A number of relatively recent control functions, including motor vehicle longitudinal control (ACC) which maintains a constant distance from a vehicle traveling in front, require, above and beyond the pure possibility of setting an analog current, particularly precise current control since the smallest deviations from the desired current value bring about perceptible differences in the brake pressure which is set, with the result that precise ACC control with corresponding comfort is no longer possible. In addition, even small differences between the pressure which is set at the front axle and the rear axle during a relatively long period of ACC control can lead to a failure of the brake function of an axle. In particular, relatively low currents in the range from approximately 100 to 400 mA should have a high level of precision since these currents are required to set small pressure differences such as are typical for longitudinal control.
In the case of PWM stages which are embodied according to the previously mentioned patent applications WO 02/058967 A2 (P 10057) and WO 03/039904 A2 (P 10253) there is therefore need to improve the precision of PWM current control still further. In a PWM mentioned controller according to the prior art, considered in general terms an inductive load (for example valve solenoid) is actuated in the general application case of brake control. The inductive load has a specific inductance L and an ohmic resistance R. A time constant of the load L/R can be defined from the inductance L. Depending on this time constant and the pulse-width-modulation frequency which is aimed at a typical profile of the current IL due to the inductive load plotted against the time t is obtained as indicated in FIG. 1. As a result of the use of an A/D converter which is used repeatedly for measuring current in different PWM channels, the current cannot be determined at a plurality of points of the current profile in FIG. 1. The current is therefore measured at specific times (time-discrete measurements), as described in the documents cited above. The current value which is determined in this way deviates considerably, depending on the measuring time, from the mean value of the current which is to be actually determined for the PWM controller. This deviation from the mean value is also referred to below as form error. If, as illustrated in FIG. 2, the current value is, for example, measured regularly in the center of the switch-on phase at the time tON/2, the form error which is illustrated in FIG. 2 is produced as a difference between the measured value and the mean value.
However, the form error is not only influenced by the measuring time of the discrete measurement of current but also by other operating parameters of the PWM controller such as, for example, the high side voltage which is present on the load and by the temperature-dependent ohmic resistance of the load at the particular time. In particular, integrated analog circuits reach a high absolute precision level only at very high cost. Although, for example, differential circuit technologies known per se and trimming techniques which are known per se permit a certain degree of independence from technological variations and temperature effects, there are limits on these methods owing to the high degree of expenditure. Trimming the circuit by means of the temperature would take a very long time during fabrication and is therefore less advantageous in terms of fabrication with high production numbers.
In order to measure current, an arrangement composed of a sense FET in conjunction with a respectively assigned sense amplifier is used in the PWM stages according to the patent applications WO 02/058967 A2 (P 10057) and WO 03/039904 A2 (P 10253) which have already been mentioned. The sense FET which is used in this arrangement typically has a temperature-dependent switch-on resistance which already leads to an extremely high measurement error at least at currents in the mA range in conjunction with an offset error which is usually present in the sense amplifier.
The current measuring principle which is illustrated in FIG. 2 requires a minimum value for the switch-on period of the PWM signal for a current value to be able to be sensed under all peripheral conditions in every period. The consequence of this minimum value is that a minimum current, below which control is no longer possible, results depending on the ohmic resistance of the solenoid, the high side voltage at the inductance and the PWM frequency which is set. In the typical application case of an ACC controller for motor vehicles, it is therefore possible, for example, only to apply currents up to a minimum of 200 mA. However, ACC-optimized current/brake pressure characteristic curves of a valve solenoid usually require lower currents down to approximately 100 mA.
The resolution of a PWM current controller determines the precision levels with which currents can be set. This depends essentially on the maximum current which can be set and on the resolution of the A/D converter which is provided for measurement of the actual value of the current.